Entrainment dynamics of buoyant jets in a stably stratified environment

Abstract: Entrainment characteristics of a pure jet and buoyant jets in a stably-stratified ambient are compared with the help of laboratory experiments employing simultaneous particle image velocimetry and planar laser induced fluorescence techniques. For the buoyant jet, two cases of background stratification are considered, N = 0.4 s and 0.6 s, where N is the buoyancy frequency. Evolution of volume flux, Q, momentum flux, M, buoyancy flux, F, characteristic velocity, w, width, d, and buoyancy, b with axial distance is quantified that helps in understanding the mean flow characteristics. Subsequently, two different methods are used for computing the entrainment coefficient, α; namely the standard entrainment hypothesis based on the mass conservation equation and energy-consistent entrainment relation proposed by van Reeuwijk and Craske (J Fluid Mech 782:333–355, 2015). It is observed that entrainment coefficient is constant for the pure jet (α≈ 0.1) up until the point where the upper horizontal boundary starts to influence the flow. The entrainment coefficient for buoyant jets, α, is not constant and varies with axial location before starting to detrain near the neutral layer. Near the source, α≈ 0.12 for both the values of N, while away from the source, N = 0.6 s exhibits a higher value of α≈ 0.15 in comparison to α≈ 0.13 for N = 0.4 s. During detrainment near the neutral layer, α≈ – 0.2 for N = 0.4 s and α≈ – 0.3 for N = 0.6 s . Importantly, close to the source, α from standard entrainment hypothesis and energy-consistent relation are in reasonable match for pure jet and buoyant jets. However, far away from the source, the energy-consistent relation is ineffective in quantifying the entrainment coefficient in the pure jet and detrainment in buoyant jets. We propose ways in which the energy-consistent relation could be reconciled with standard entrainment hypothesis in the far-field region. Article Highlights: Entrainment coefficient stays invariant for jets till the finite size of the domain in the axial direction disrupts this feature.Entrainment coefficient for buoyant jets evolving in a stratified ambient varies with axial distance followed by detrainment beyond the neutral layer.The existing entrainment relation performs reasonably well in the momentum dominated region but performs poorly when the finite size of the domain affects the flow for pure jet and when the flow is buoyancy dominated for the case of buoyant jets.